From: Yang Shi <shy828301@gmail.com>
To: ziy@nvidia.com, kirill.shutemov@linux.intel.com,
wangyugui@e16-tech.com, hughd@google.com,
akpm@linux-foundation.org
Cc: shy828301@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: [v2 PATCH] mm: thp: check total_mapcount instead of page_mapcount
Date: Thu, 13 May 2021 14:23:34 -0700 [thread overview]
Message-ID: <20210513212334.217424-1-shy828301@gmail.com> (raw)
When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
return false positive for PTE-mapped THP since page_mapcount() is used
to check if the THP is unmapped, but it just checks compound mapount and
head page's mapcount. If the THP is PTE-mapped and head page is not
mapped, it may return false positive.
Use total_mapcount() instead of page_mapcount() for try_to_unmap() and
do so for the VM_BUG_ON_PAGE in split_huge_page_to_list as well.
This changed the semantic of try_to_unmap(), but I don't see there is
any usecase that expects try_to_unmap() just unmap one subpage of a huge
page. So using page_mapcount() seems like a bug.
[1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@e16-tech.com/
Signed-off-by: Yang Shi <shy828301@gmail.com>
---
v2: Removed dead code and updated the comment of try_to_unmap() per Zi
Yan.
mm/huge_memory.c | 11 +----------
mm/rmap.c | 10 ++++++----
2 files changed, 7 insertions(+), 14 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 63ed6b25deaa..3b08b9ba1578 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2348,7 +2348,6 @@ static void unmap_page(struct page *page)
ttu_flags |= TTU_SPLIT_FREEZE;
unmap_success = try_to_unmap(page, ttu_flags);
- VM_BUG_ON_PAGE(!unmap_success, page);
}
static void remap_page(struct page *page, unsigned int nr)
@@ -2718,7 +2717,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
}
unmap_page(head);
- VM_BUG_ON_PAGE(compound_mapcount(head), head);
+ VM_BUG_ON_PAGE(total_mapcount(head), head);
/* block interrupt reentry in xa_lock and spinlock */
local_irq_disable();
@@ -2758,14 +2757,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
__split_huge_page(page, list, end);
ret = 0;
} else {
- if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
- pr_alert("total_mapcount: %u, page_count(): %u\n",
- mapcount, count);
- if (PageTail(page))
- dump_page(head, NULL);
- dump_page(page, "total_mapcount(head) > 0");
- BUG();
- }
spin_unlock(&ds_queue->split_queue_lock);
fail: if (mapping)
xa_unlock(&mapping->i_pages);
diff --git a/mm/rmap.c b/mm/rmap.c
index 693a610e181d..f52825b1330d 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1742,12 +1742,14 @@ static int page_not_mapped(struct page *page)
}
/**
- * try_to_unmap - try to remove all page table mappings to a page
- * @page: the page to get unmapped
+ * try_to_unmap - try to remove all page table mappings to a page and the
+ * compound page it belongs to
+ * @page: the page or the subpages of compound page to get unmapped
* @flags: action and flags
*
* Tries to remove all the page table entries which are mapping this
- * page, used in the pageout path. Caller must hold the page lock.
+ * page and the compound page it belongs to, used in the pageout path.
+ * Caller must hold the page lock.
*
* If unmap is successful, return true. Otherwise, false.
*/
@@ -1777,7 +1779,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
else
rmap_walk(page, &rwc);
- return !page_mapcount(page) ? true : false;
+ return !total_mapcount(page) ? true : false;
}
/**
--
2.26.2
next reply other threads:[~2021-05-13 21:23 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-13 21:23 Yang Shi [this message]
2021-05-14 13:50 ` [v2 PATCH] mm: thp: check total_mapcount instead of page_mapcount Zi Yan
2021-05-21 5:06 ` Hugh Dickins
2021-05-21 17:16 ` Yang Shi
2021-05-21 19:27 ` Yang Shi
2021-05-21 23:17 ` Hugh Dickins
2021-05-22 0:36 ` Yang Shi
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